There is a microwave plasma processing apparatus which uses high density plasma excited by a microwave electric field. The microwave plasma processing apparatus includes, for example, a processing container for use in performing a plasma processing on a substrate to be processed, a microwave generator configured to generate microwaves which convert a processing gas into plasma within the processing container, and a waveguide configured to guide the microwaves generated by the microwave generator into the processing container. Further, the microwave plasma processing apparatus includes a planar antenna provided with a plurality of slots which transmit the microwaves guided by the waveguide. In the microwave plasma processing apparatus, the microwaves are radiated from a slot antenna into the processing container to ionize the processing gas within the processing container, thereby exciting the plasma.
However, in the microwave plasma processing apparatus, in order to uniformly perform the plasma processing on the entire surface of an object to be processed, it is requested that the plasma excited by the microwaves be uniformly distributed in the processing space within the processing container. In this regard, a technology is known in which a distributor is installed at a rear end side of a microwave generator to distribute the microwaves supplied from the microwave generator to a plurality of waveguides using a prescribed distribution ratio by the distributor and the distributed microwaves are radiated from an antenna to a central side and a peripheral side of the processing space. See, e.g., Japanese Laid-Open Patent Publication Nos. H09-63793, H03-191074, 2007-213994, and 2011-014542.